1. Field of the Invention
The present invention relates to an auxiliary chamber type internal combustion engine and, more particularly, to a structure for the combustion chamber of the auxiliary chamber type internal combustion engine. 2. Description of the Prior Art
One example of the combustion chamber of a swirl chamber type Diesel engine of the prior art is shown in FIG. 1. As shown, an auxiliary combustion chamber 2 is formed by recessing a cylinder head 4. Specifically, the auxiliary combustion chamber 2 is formed of the recess in the cylinder head 4 and an auxiliary chamber plug 9 which is fitted in that recess from the lower side of the cylinder head 4. The plug 9 is fixed in the cylinder head 4 and between the cylinder head 4 and a cylinder 8 through a cylinder head gasket 10. Moreover, the lower faces of the auxiliary chamber plug 9 and the cylinder head 4 extend in a common plane.
The auxiliary combustion chamber 2 has its upper portion formed into a semispherical shape and its lower portion formed into a frusto-conical shape, a cylindrical shape or the like. FIG. 1 shows the auxiliary combustion chamber 2 having a lower portion of the frusto-conical shape. The auxiliary combustion chamber 2 is arranged, if necessary, with a fuel injection valve 5 and a glow plug 6 for preheating the inside of the auxiliary combustion chamber 2 at the start of the engine. The auxiliary combustion chamber 2 has communication through an auxiliary chamber injection port 3 with a main combustion chamber 1 which is defined by the top face of a piston 7, the inner face of the cylinder 8 and the lower face of the cylinder head 4. The auxiliary chamber injection port 3 is formed wholly or mostly in the auxiliary chamber plug 9. The former case is shown in FIG. 1. As shown, moreover, the auxiliary chamber injection port 3 has a straight axis and is at a constant angle .theta.to a perpendicular to the axis of the auxiliary combustion A--A.
At the compression stroke of the engine, the air in the combustion chamber 1 is compressed by the piston 7 to flow through the auxiliary chamber injection port 3 into the auxiliary combustion chamber 2 thereby to generate a swirl S. When fuel is injected from the fuel injection valve 5 along that swirl S, it is swirled in the auxiliary combustion chamber 2 by the swirl S so that it is mixed with the air and ignited and burned. The mixing of the unburned fuel, which is injected from the auxiliary combustion chamber 2, with the air in the main combustion chamber 1 is undergone by the gas jet from the auxiliary combustion chamber 2. The jet spurting from the auxiliary combustion chamber 2 arrives to impinge upon the cylinder wall diametrically opposed to the auxiliary combustion chamber 2 with respect to a cylinder center line B--B. After this impingement, the fuel is scattered along the surface of the cylinder wall.
However, the aforementioned structure of the prior art has the following defects.
In order to improve formation and combustion of the air-fuel mixture in the main combustion chamber 1, the jet has to reach the aforementioned cylinder wall for a short time period. In the case of a small-sized swirl chamber type Diesel engine, generally speaking, the arrangement of intake and exhaust valves and so on raises a structural limit in that the auxiliary chamber is formed close to the cylinder center line B--B. Because of this limit, the auxiliary chamber injection port 3 has its effective area reduced to increase the jet velocity so that its throttle loss and the heat loss in the main combustion chamber are high.
As is apparent from the experimental results (in which measured values are indicated by solid circles) of the jet characteristics in the main combustion chamber of the auxiliary chamber type engine, as shown in FIG. 2, the effective area of the auxiliary chamber injection port 3 can be enlarged because the jet penetration into the main combustion chamber 1 is increased for a small auxiliary chamber injection port angle .theta..
If, however, this angle 0 is reduced in such a straight auxiliary chamber injection port 3 of the prior art as is shown in FIGS. 1 and 3, the angular difference of (180-.theta.).degree. between the direction of the swirl in the auxiliary combustion chamber 2 and the direction of the gas jet into the main combustion chamber 1 becomes large in the gas jet from the auxiliary combustion chamber 2 into the main combustion chamber 1 at the expansion stroke, as is apparent from the experimental results (in which measured values are indicated by solid circles) of the flow coefficient of the auxiliary chamber injection port, as shown in FIGS. 4(a) to (c). This makes it difficult for the gas to flow into the main combustion chamber 1 (as a result of a reduction in the auxiliary chamber injection port flow coefficient) to remarkably enlarge the throttle loss of the auxiliary chamber injection port or the flow coefficient of the auxiliary injection port thereby to invite instability. The latter phenomenon that the auxiliary chamber injection port flow coefficient is remarkably enlarged is caused mainly because the effective area F of the open end of the auxiliary chamber injection port 3 at the side of the auxiliary chamber is remarkably increased if the auxiliary chamber injection port angle .theta. is small, as shown in FIGS. 4(a) to (c).
FIG. 5 is a sectional view showing another example of the prior art. As shown, an open end of the auxiliary chamber injection port 3 at the side of the auxiliary combustion chamber is located at the side of the cylinder center line B--B with respect to an auxiliary combustion chamber center line A--A. Moreover, the outflow angle .theta..sub.1 of the open end of the auxiliary chamber injection port 3 at the side of the main combustion chamber 1 is made smaller than the outflow angle .theta..sub.2 of the open end of the auxiliary chamber injection port 3 at the side of the auxiliary combustion chamber 2.
When the fuel injection nozzle 5 is disposed in the position of FIG. 3, a swirling radius r.sub.s in the auxiliary chamber is small in a case where the outflow angle .theta..sub.2 is small, as indicated by broken lines in FIG. 5, or the swirling direction of the swirl in the auxiliary chamber and the fuel injection direction are reversed, as indicated by solid lines in FIG. 5, to degrade the mixing of the fuel with the air and the combustion of the mixture in the auxiliary combustion chamber 2.
This problem is solved if the fuel injection nozzle 5 is positioned, as shown in FIG. 5, and if the auxiliary chamber injection port 3 is so formed as is shown by the solid lines. Despite this fact, however, the difference of .DELTA..theta.(=.theta..sub.1 -.theta..sub.2) between the auxiliary chamber injection port angles at the sides of the auxiliary combustion chamber 2 and the main combustion chamber 1 becomes large. As a result, the flow loss is increased, as shown in FIG. 6, to weaken the swirl in the auxiliary combustion chamber 2 and to drop the velocity of the jet in the main combustion chamber 1 so that the mixing of the fuel with the air and the combustion of the fuel are degraded. In this case, moreover, the structure of the cylinder head has to be modified so much that the production cost is raised.
Turning to FIG. 7, there is shown a structure in which the auxiliary chamber injection port 3 is positioned such that its opening at the side of the auxiliary combustion chamber 2 is formed oppositely of the cylinder center line B-B with respect to the auxiliary combustion chamber center line A--A, such that the outflow angle .theta..sub.2 at the open end of the auxiliary chamber injection port 3 at the side of the auxiliary combustion chamber 2 is made larger than the outflow angle .theta..sub.1 at the open end of the same auxiliary chamber injection port 3 at the side of the main combustion chamber, and such that the opening of the same auxiliary chamber injection port 3 at the side of the main combustion chamber is formed in the side face of the auxiliary chamber plug 9, and in which the cylinder head 4 has its lower face reamed to form a part of the auxiliary chamber injection port 3. Of the auxiliary injection ports 3 of FIG. 7, one having an outflow angle .theta..sub.2 &gt;90.degree. at its open end at the side of the auxiliary combustion chamber 2 is shown by broken lines, whereas the other having the same angle .theta..sub.2 &lt;90.degree. is shown by solid lines. From the structural requirement of the aforementioned auxiliary chamber injection port 3, the outflow angle .theta..sub.1 at the open end at the side of the main combustion chamber 1 has to be made very small whereas the same outflow angle .theta..sub.2 at the side of the auxiliary combustion chamber 2 has to be made very large. Moreover, the axis of the auxiliary chamber injection port 3 is substantially bent. This makes it impossible to expect much improvement in the mixing of the fuel with the air and the combustion of the fuel because the flow loss in the auxiliary chamber injection port 3 is increased which weakens the swirl in the auxiliary combustion chamber 2 and drops the velocity of the jet in the main combustion chamber 1. For the aforementioned case of .theta..sub.2 &gt;90.degree., moreover, the swirling radius r.sub.s of the swirl in the auxiliary chamber becomes small so that the intensity of the swirl in the auxiliary combustion chamber 2 is reduced which degrades the mixing of the fuel with the air and the combustion of the fuel. From the structural requirement of the auxiliary chamber injection port 3 and the lower face of the cylinder head 4, still moreover, their parts have to be modified so much that the production cost is raised and that a problem arises in degradation in the durability of the auxiliary chamber plug 9.